This invention relates generally to an electro-optical device and, more particularly, to a surface-emitting semiconductor laser.
Conventional vertical cavity surface-emitting lasers (VCSELs) typically have two flat resonator cavity mirrors formed onto the two outer sides of a layered quantum-well gain structure, and are significantly limited in single spatial-mode output power, typically a few milliwatts. While greater optical power can be achieved from conventional VCSEL devices by using larger emitting areas, such a large aperture device is not particularly practical for commercial manufacture or use, and produces an output which is typically distributed across many higher order spatial modes. Several schemes have been proposed for increasing single-model output power from surface-emitting devices. One approach is to replace one of the mirrors adjacent the active region of a conventional VCSEL device with a more distant reflector whose curvature and spacing from the active region preferentially supports a fundamental spatial mode. Such a device architecture is called a VECSEL (Vertical Extended Cavity Surface Emitting Laser).
xe2x80x9cHigh single-transverse mode output from external-cavity surface emitting laser diodesxe2x80x9d, M. A. Hadley, G. C. Wilson. K. Y. Lau and J. S. Smith, Applied Phys. Letters, Vol. 63, No. 12, 20 Sep. 1993, pp. 1607-1609, describes a triple-mirror, coupled-cavity device with an epitaxial p-type bottom Bragg mirror and undoped quantum-well gain structure grown on an external p-type substrate followed by an n-type coupled cavity intermediate mirror. The medium between the coupled cavity intermediate n-type mirror and the output coupler was air. Since any heat produced in the active gain region must be removed through the relatively thick p-type substrate, the practical output power from such a device is limited to about 100 mW for pulsed operation and to only a few mW for continuous (xe2x80x9ccwxe2x80x9d) operation.
xe2x80x9cAngular filtering of spatial modes in a vertical-cavity surface-emitting laser by a Fabry-Perot xc3xa9alon,xe2x80x9d by Guoqiang Chen, James R. Leger and Anand Gopinath. Applied Physics Letters, Vol. 74 No. 8, Feb. 22, 1999, pp. 1069-1071, describes an integrated Fabry-Perot xc3xa9alon formed of GaAs between a reduced bottom mirror stack of the VCSEL and a backside dielectric mirror, to thereby form an integrated coupled oscillator in which the angular plane-wave spectra of the higher-order modes have been spatially filtered out. No electrode configurations are shown or described and it is not apparent how that device could be electrically excited to produce high levels of output power.
My commonly assigned PCT publication WO 98/43329 describes a novel architecture for an electrically-excited vertical extended cavity surface emitting laser (VECSEL) device that enables the output power emitted in the single, lowest order TEM00 spatial mode to be scaled upwards more than an order of magnitude beyond that achievable with other known VECSELs, while being much more practical and manufacturable than was previously achievable. In that device, the quantum-well gain layers were grown directly on the bottom surface of the n-type substrate; this growth was then followed by the usual highly-reflecting p-type DBR cavity mirror. The laser cavity was formed by depositing an anti-reflective coating on the top surface of the n-type substrate, and placing a concave external mirror away from the substrate with the mirror""s optical axis oriented perpendicular to the plane of the substrate, such that the n-type substrate was located physically and optically within the laser cavity. Such an internal substrate configuration not only provides structural integrity and ease of manufacture (especially when the external mirror is formed on or otherwise placed directly on top of the inverted substrate), it also facilitates an electrode placement that is optimal for efficient electrical excitation and operation in the TEM00 mode with a larger aperture and high output power levels than would otherwise be possible. However, especially in an electrically pumped device with a relatively thick substrate inside the laser cavity, increasing the doping of the substrate (desirable to minimize carrier crowding and electrical resistance) also increases the optical loss at the laser wavelength and the overall efficiency of the device is correspondingly reduced.
An overall objective of the present invention is to provide a surface emitting coupled cavity semiconductor laser device capable of producing one or more desired spatial modes at higher power levels and with greater device efficiency than would be feasible with known prior art VCSELs and VECSELs.
In accordance with the broader aspects of the present invention, an undoped gain region sandwiched between a nominally 100% reflective bottom Bragg mirror and an intermediate partially reflecting Bragg mirror is formed on a bottom lower surface of a supporting substrate, to thereby provide the first (xe2x80x9cactivexe2x80x9d) resonator cavity of a high power coupled cavity surface emitting VECSEL laser device. The bottom mirror is preferably in direct thermal contact with an external heat sink for maximum heat removal effectiveness. The reflectivity of the intermediate mirror is kept low enough so that laser oscillation within the first active gain region will not will not occur without optical feedback from a second, passive resonator cavity, formed by the intermediate mirror and an external mirror contiguous to the upper surface of the VECSEL substrate. Thus, the substrate is entirely outside the first active resonator cavity but is contained within a second (xe2x80x9cpassivexe2x80x9d) resonator cavity defined by the intermediate mirror and a partially reflecting output mirror. This second passive resonator cavity is directly coupled optically to the first active resonator cavity, and is designed to effectively increase the gain within the first active resonator cavity above the laser threshold, and/or to reduce the threshold for laser action in the first active resonator cavity, such that the output of the device is largely determined by the optical feedback from the second passive resonator cavity. Since the substrate is contained only in the second passive resonator cavity, and since the intermediate mirror forming this second passive resonator cavity typically has a transmissivity of only a few percent, the optical laser power in the second cavity is only a small fraction of the laser intensity circulating in the first active resonator cavity; therefore the substrate sees only a correspondingly small percentage of the light intensity energy that is circulating in the gain region. Thus any loss or other undesired effects caused by light intensity energy passing through the substrate are only that same small percentage that they would have been had that same substrate been placed in the same resonant cavity as the active gain region.
In a preferred embodiment, an electrically-excited coupled-cavity. VECSEL electrically excited coupled cavity VCSEL utilizes an n-type semiconductor substrate with a partially reflective intermediate reflector (preferably an n-type Bragg mirror) grown on a bottom surface of the substrate. An undoped gain medium is grown or positioned under the intermediate reflector, and a bottom reflector is grown or positioned under the gain medium, to thereby form a first an active: resonant cavity containing having an active gain region. The bottom reflector is preferably a p-type Bragg mirror having a reflectivity of almost 100 %, which is soldered to or otherwise placed in thermal contact with an external heat sink. A second passive resonator cavity is formed by the partially-transmitting intermediate cavity mirror grown on the bottom surface of the n-type substrate, and a partially-transmitting output cavity mirror, positioned externally above the upper surface of the substrate. The output mirror is positioned above the substrate at the opposite side of the p-type Bragg mirror and defines a passive resonant cavity. This second passive resonator cavity is designed to control the spatial and frequency characteristics of the optical feedback to, and thus the laser oscillation within, the first active resonant cavity. It in effect functions as a spatial filter, with the external output cavity mirror preferably configured (curvature, reflectivity, and distance from the intermediate reflector) to limit the laser to confine the resonant radiation within the second passive resonator cavity to a single fundamental mode; since the mode of any laser output from the first active resonator cavity is determined by the mode of the feedback from the second passive resonator cavity, the output spatial mode from the overall device is essentially confined to that single fundamental mode.
Such a novel VECSEL structure is particularly advantageous when the electrical current is applied to an external electrode and must pass through a conductive substrate in order to reach the active gain region. Since the active gain region is in a first one cavity and the conductive substrate is in second another cavity, the substrate can have a substantially higher doping level and/or a substantially associated lower electrical resistance than would otherwise be possible. The electrode configuration is preferably similar to that described in my referenced 20 International patent publication, with the disk shaped bottom electrode formed by an oxide current aperture between the bottom mirror and the heat sink and with the annular top electrode formed on the top surface of the substrate (above or surrounding the AR coating), to thereby define a cylindrical electrically excited primary gain region surrounded by an annular secondary gain region.
In accordance with the method aspects of the present invention, the first active resonant cavity is epitaxially grown on the bottom surface of the substrate. The top surface of the substrate is provided with an anti-reflective coating and an external output mirror configured to control the desired mode or modes of the laser energy resonating both in the second passive resonant passive and in the first active cavity. In the preferred embodiment the external mirror is separated from the substrate and is configured to provide the desired fundamental mode output. In an alternative embodiment that takes particular advantage of the coupled-cavity configuration to reduce losses within the second passive cavity, the substrate may occupy the full extent of the second passive cavity and its top surface may be configured by binary optics techniques prior to depositing the required upper electrode and top reflector, to thereby produce monolithic fully integrated coupled cavity device.
Optionally a non-linear frequency doubling material may be included inside the second passive resonant cavity to thereby convert or reduce the output wavelength from the longer wavelengths associated with typical semiconductor laser materials, such as GaAs and GaInAs, to the shorter wavelengths necessary or desirable for various medical, materials processing, and display applications. In that case, the reflectivity characteristics of the various optical components are preferably chosen to favor the feedback of the unconverted fundamental wavelength back towards the active gain region and the output of any already converted harmonics through the output mirror.
As another option, a polarizing element which selectively favors a desired polarization orientation may be included within the second passive resonant cavity. Such a polarizing element may be in the form of a two-dimensional grid of conductive lines located at an anti-node of the optical energy resonating within the second passive resonant cavity to thereby absorb polarization parallel to those lines, and may be conveniently formed on the upper surface of the substrate adjacent to the anti-reflection layer.
Alternatively a saturable absorber or other suitable mode-locking means may be included within the second passive resonator cavity to provide a high peak power output pulse.
In yet another optional embodiment, the second passive resonator cavity is integrated with one end of a single mode optical fiber by means of a focusing lens element and the reflector defining the upper end of the second passive resonant cavity is in the form of a distributed Bragg reflector formed by longitudinal variations in the refractive index of the fiber.
A plurality of coupled cavity vertical extended cavity surface emitting lasers (VECSELs) devices having different modes and/or frequencies may be fabricated in one- or two-dimensional arrays, to thereby provide a wideband transmission source for multimode optical fiber transmission systems and/or to provide a 3-color light source for a projection display. Alternatively the individual devices of such an array may be operated coherently by means of a shared passive external resonator cavity to provide a coherent single mode output having an even higher power than would otherwise be possible. Such a device would use, for example, a spatial filter in the passive cavity to force all elements of the array to emit in phase.
An additional advantage of a coupled cavity device constructed in accordance with the present invention is that the output laser wavelength is determined by the Fabry-Perot resonance frequency of the active cavity. This wavelength tunes with temperature at the rate of about 0.07 nm per degree Centigrade for GaInAs type devices operating in the 980 nm wavelength region, thereby providing a convenient tuning-mechanism for certain applications requiring a variable wavelength tunable output, in discrete jumps essentially corresponding to the possible resonances within the second passive cavity.
Although the hereinafter-described preferred embodiment utilizes electrical excitation and an n-type doped substrate, many aspects of the invention are also applicable to optical or e-beam excitation, and to the use of n-type materials for the Bragg mirrors at both ends of the first active resonator cavity, with one or more Esaki diodes placed at resonant nodes inside the first active resonator cavity,